Apparatus and methods are known for inducing therapeutic hyperthermia in biological tissue by inductive, radiant, contact and joulean heating methods. Inductive methods, such as described in U.S. Pat. Nos. 5,251,645 and 4,679,561, heat a volume of tissue located within a body cavity by passing high frequency electromagnetic radiation through tissue positioned between two external electrodes located near or in contact with the patient's skin. Heating is achieved due to the interaction of the changing electromagnetic field within the tissue.
A drawback of the foregoing devices is that this approach to therapeutic hyperthermia heats a relatively large volume of tissue to elevated temperatures for extended periods of time. Typically, tissue is heated to temperatures of 6 to 10.degree. C. above normal body temperature, with heating sources operating in a range from ultrasonic frequencies to microwave frequencies, for periods of 20 minutes or more to achieve a desired degree of necrosis. Such devices generally do not allow the volume of tissue to be well defined, resulting in either insufficient necrosis or excessive necrosis of surrounding healthy tissue. Consequently, diffuse and prolonged heating of tissue is often combined with chemotherapy or radiation therapy modalities.
Other previously known methods and apparatus include therapeutic heating of tissue using radiant sources as described in U.S. Pat. Nos. 5,284,144, 4,872,458, and 4,737,628. Radiant sources, such as lasers, produce localized heating of tissue, but do not permit the effected volume to be predetermined a priori.
In addition, contact heating methods and apparatus have been used for inducing therapeutic hyperthermia, such as described in U.S. Pat. Nos. 4,979,518, 4,860,744, 4,658,836, and 4,520,249. Contact heating methods are not well suited to heating a defined volume of material, such as a tumor mass, due to thermal gradient effects. For example, to heat a spherical volume of the tissue having a diameter of 2 cm to at least 60.degree. C., a single heating element could be inserted along a diameter of the sphere. To raise the perimeter of the spherical volume to 60.degree. C., the central regions must be raised to a higher temperature than the periphery to produce an adequate thermal gradient, as may be demonstrated using well-known thermal conduction equations.
A disadvantage of the often high thermal gradient across the spherical volume, depending on the tissue type and conductivity, is the unwanted evolution of steam, formation of eschar, and unwanted preferential heat transfer along the cannula support shaft, thereby detrimentally effecting healthy tissue outside the target region.
In order to overcome the foregoing limitations, dispersed contact heating methods also have been developed. For example, small spheres or wire segments of ferromagnetic alloys have been inserted into tumors in the brain and other tissue and heated to an auto-regulating temperature (i.e., the Curie temperature of the alloy) by an externally applied electromagnetic field. The resulting eddy current heating causes hyperthermia in the tissue immediately surrounding the small spheres or wire segments.
Yet another approach to therapeutic hyperthermia, referred to as "electrosurgery," utilizes heating produced by the flow of electrical current through tissue, such as described in U.S. Pat. Nos. 5,599,346, 5,599,345, 5,486,161, 5,472,441, 5,458,597, 5,536,267, 5,507,743, 4,846,196, 4,121,592, and 4,016,886. Electrosurgery generally employs either a monopolar or bipolar modality. In the monopolar mode, electric current is conducted between a relatively small active electrode and a large return electrode located at a distance from the active electrode. Because in the monopolar mode the current density in tissue decreases as the square of the distance from the active electrode, it is difficult to obtain necrosis of a predetermined volume of tissue.
The bipolar mode of electrosurgical (joulean) heating, such as described in U.S. Pat. Nos. 5,122,137, 4,920,978, 4,919,138, and 4,821,725, involves passing current between tissue disposed between two electrodes of similar surface area. Like monopolar heating, however, the capability to heat tissue in a precise manner requires that the region of tissue to be exposed to therapeutic hyperthermia be accurately defined in terms of both location and dimensions.
In a further attempt to address the disadvantages of previously known devices, methods have been developed for monitoring and/or controlling the progress or extent of therapeutic heating (or cooling) of tissue. These techniques include measurement of tissue temperature in contact with the device, such as described in U.S. Pat. Nos. 5,122,137, 4,776,334, and 4,016,866, and direct measurement of tissue impedance, such as described in U.S. Pat. Nos. 5,069,223 and 4,140,109. A limitation of such previously known devices is the need for specially designed or dedicated control systems and/or power supplies capable of measuring the specific parameter of interest (e.g., temperature or electrical impedance). This requirement for specialized equipment often poses budgetary problems in health care institutions, thus limiting widespread acceptance of such devices.
Additionally, measurement of actual tissue impedance is complicated by the wide range of variation in the electrical properties of biological tissue depending on the fatty tissue content and vascularity of the tissue. Further, tissue temperature measurements may be influenced by the distance between the temperature sensor and the working surface of the device, often resulting in underestimation of temperatures for more distal regions of tissue. In particular, the use of electrosurgical heating methods can lead to tissue heating effects which may be several or tens of millimeters from the working surface, well beyond the range of a temperature sensor mounted near the working surface.
Another important limitation of previously known devices and methods is the necessity of an invasive procedure, following the biopsy procedure, to treat abnormal or diseased tissue. For example, breast tumors or other abnormal tissue masses may be first identified by palpation, radiography, thermography and/or ultrasonography. Once a tumor is detected, a biopsy needle is used to extract a tissue sample (under the guidance of radiographs, ultrasound and/or palpation), and the biopsy needle is withdrawn from the patient. If hyperthermic treatment is indicated, the patient is subsequently exposed to a separate procedure (often invasive) which may be hours, days, weeks or longer after the initial invasive needle biopsy procedure.
In view of the foregoing, it would therefore be desirable to provide methods and apparatus, for use with existing tumor imaging techniques, capable of applying therapeutic hyperthermia, in situ, to any tumor which may be identified using minimally invasive procedures.
It would further be desirable to provide methods and apparatus for effecting complete necrosis of an identified tumor mass of predetermined volume, with minimal damage to surrounding healthy tissue.
It also would be desirable to provide methods and apparatus for effecting treatment of a tumor in single procedure that achieves therapeutic hyperthermia of a predetermined volume of tissue within a brief period of time (e.g., several seconds to tens of seconds).
It would still further be desirable to provide methods and apparatus for effecting the treatment of a tumor promptly after completion of a biopsy procedure, thus permitting utilization of the vectoring biopsy needle guide cannula and/or tumor imaging techniques to facilitate accurate positioning of a tissue cauterization device.
It would also be desirable to provide methods and apparatus for treating tumors wherein an energy applicator could be tailored to the size and shape of the tumor, as quantified, for example, using tumor imaging techniques.
It would be desirable to provide methods and apparatus for treating abnormal tissue that include an automatic shut-off control, with suitable visual and/or audible indicators, that signal when therapy is complete, i.e., when a predetermined volume of tissue has been cauterized.
It would further be desirable to provide methods and apparatus for treating abnormal tissue that includes an expandable geometry that provides increased treatment surface area while in-situ but a relatively smaller insertion diameter, thereby reducing insertion trauma.
It yet further would be desirable to provide methods and apparatus for performing therapeutic cauterization of tissue using commonly available electrosurgical generators.